In recent years, organic emitting diode (OLED) display has been widely studied and used as new generation display. This is because OLED display has many advantages, such as high brightness, high luminous efficiency, wide viewing angle and low power consumption. There are two kinds of driving method of OLED display, namely passive matrix OLED (PMOLED) and active matrix OLED (AMOLED).
Although PMOLED has the merit of low cost, there is cross talk phenomenon which hinders the implementation of high resolution display using PMOLED. And due to large driving current, the lifetime of PMOLED is decreased.
In contrast, for AMOLED, there are different numbers of transistors acting as current source in each pixel. Thus crosstalk can be avoided, and the driving current of OLED is small. Consequently, not only the power consumption is low, but also the lifetime of OLED can be extended, and high resolution display can be achieved. Meanwhile, it is easier to obtain large area display with high gray level by using AMOLED technology.
The conventional AMOLED pixel circuit consists of two thin film transistors (TFTs) and one storage capacitor. As shown in FIG. 1, the pixel circuit includes a driving TFT12, a switching TFT11, a storage capacitor 13 and an emitting device OLED14. The signal of the scan signal line can control the switching TFT12. Data signals of the data signal line 16 are sampling and supplied to gate electrode of the driving TFT11. The driving TFT11 generates the current needed by the OLED14 corresponding to the required gray scale. And the gray scale information is stored in a storage capacitor 13, and the storage capacitor 13 maintains the sampled data until the next frame.
      I    OLED    =            1      2        ⁢          μ      n        ⁢          C      ox        ⁢          W      L        ⁢                  (                              V            G                    -                      V            OLED                    -                      V            TH                          )            2      
where μn, Cox, W/L are the field-effect mobility and gate capacitance per unit area and width/length ratio of the driving TFT11, respectively. And VG represents the voltage of the gate of the driving TFT11. And VOLED represents the voltage of the anodic of the OLED14 in emitting. And VTH represents the threshold voltage of the driving TFT11. Although the circuit structure is simple, the threshold voltage shift of the driving TFT11 and the degradation of OLED14 lead to the increase of VOLED over time. In the case of poly-silicon TFTs, due to the non-uniformity of threshold voltage of the driving TFT11 in the panel, the current of OLED14 changes with time or location, which leads to the uniformity issue in the display.
At present, there are two types of compensation method within pixel array, namely current mode and voltage mode. Although the current mode pixel circuit allows an accurate compensation, a long settling time is needed, especially in the case of small programming current and large parasitic capacitance on the data line. This severely limits the application of the current mode pixel circuit in a large area display with high resolution. Compensation accuracy of the voltage mode pixel circuit is worse than that of the current mode pixel circuit, and structure or/and drive signals of the voltage mode pixel circuit are relatively complicated. But the driving speed of the voltage mode pixel circuit is fast.
Nowadays, for most of the voltage type pixel circuits, threshold voltage is extracted using the topology of diode-charging or discharging, as shown in FIG. 2. In this scheme, a reasonable programming time is required to accurately extract the threshold voltage of the driving TFT21. On one hand, if the programming time is too short, the discharging of storage capacitor 22 will not be completed, thus the extracted threshold voltage (voltage of node 23) is higher than the actual value. On the other hand, after the extraction of the threshold voltage of the driving TFT21 is completed, the drive transistor begins to enter the sub-threshold region, and if the programming time is too long, the storage capacitor 22 will continue to discharge through the driving TFT21, resulting in the extracted threshold voltage to be less than the actual value. In the practical application, when the threshold voltage of both the driving TFT and the emitting device OLED14 is degraded, it is difficult to accurately determine the programming time.